Treatment for coated magnesium and its alloys



Patented Jan. 13, 1942 TREATMENT-FOR am monnsromm I are armors Robert W. Buzzard, Kenslng'ton, Md.

No Drawing. Application November 27, 1940, Serial No. 367,406

3 Claims. (Cl. 148-6) (Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) This invention relates to the treatment of a corrosion resistant coating on magnesium and its alloys and one of the objects is to increase the durability and the protective characteristics of the coating.

Themethod specifically relates to treating coatings produced by chemical reaction on magnesium or magnesium alloys by subjecting, the coated metal to a second treatment which, with the first coating, forms not only a seal for the first coating but a new coating that increases the durability of the article coated to wear and improves its resistance to corrosion. The protective coating obtained by my treatment is dependent on the presence of a prime coating and .i's' gnot in itself a prime surface treatment.

The article to be treated according to this invention is washed free 01 the primary coating solution which is applied by the dip or immersion method and then is immersed in and subjected to the chemical reaction of my secondary coating and sealing solution, the temperature of which may vary-from 'room to the boiling point of the solution.

The secondary coating and sealing solution should be an aqueous solution containing (1) one or more soluble salts of chromic acid, or a mixture of chromic acid and a salt thereof soluble in the-solution, or a mixture of chromic acidand oxides soluble in a chromic acid solution so as to produce the desired salts of chromic acid; and (2) magnesium fluoride. Theconcentration of the salts of chromic acid is limitedonly by their solubility in the solution, while the magnesium fluoride being'only slightly soluble, -very minute additions in terms of percentage up to the saturation point of magnesium fluoride in the solution are effective. stances may be varied to conform with the temperature of the solution, time of application, and quality of coating desired. Good sealed coatings have been obtained by subjecting the coated metal to treatment for periods varying from 5 minutes to 60 minutes in solutions at temperatures ranging from 85 C. to

solution. I

The magnesium or magnesium alloy is first thoroughly cleaned, a clean surface being essential to the formation of good films, and then the metal is provided with an adherent coating formed integrally therewith by the chemical reaction of a solution containing nitric acid and salts oi chromic acid. The coated metal is then washed and subjected to the chemical reaction of an aqueous solution containing salts of chromic the boiling point of the acid and magnesium fluoride adjusted within the pH range of 3 through 6.5. The total concentration of chromic acid salts may vary from 1% to the point of saturation of the salts in the solution, and the magnesium fluoride may vary from minute additions in percentage up to the saturation point of this substance. The temperature of the solution may vary from room temperature to the boiling point of the solution, the preferable The concentrations of the subtemperature being substantially between 80 C. to the boiling point of the solution.

Specifically, as an example, if a coating is formed on magnesium or its alloys by dipping or immersing the metal for a period of to 2 minutes in a bath containing a water soluble dichromate, such as sodium dichromate, and nitric acid or in a bath containing chromic acid and nitric acid, its corrosion resistance will be improved it it is immersed for aperiod of 5 minutes to 60 minutes in an aqueous solution containing approximately 10% sodium dichromate saturated with magnesium fluoride, the time of treatment depending upon the temperature of the solution and the resultant coating desired, a temperature Sodium dichromate pounds- 1.5 Concentrated nitric acid pints 1.5 Water, to make gallon 1.0

The, combined coating and seal of my invention may beapplied at room temperature or even lower. However, more favorable conditions are found the salt in the solution. That is, immersions remy solution for sealing the coated magnesium. However, sodium, potassium, and ammonium chromates or dichromates are preferred because of commercial'availability.

The above mentioned substances used in my improved coating and sealing treatments are in hot solutions, in that it takes less time to com-, plete a satisfactory treatment. In like manner, time of treatment depends on concentration of.

dichrom'ate may 4 be used with the magnesium fluoride to make up I claim:

tion, since the invention comprehends treatments of the character indicated broadly above and is not to be limited except by the appendedclaims.

The invention described herein may be manufactured, used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

1. A method of producing corrosion resistant coatings on magnesium and magnesium alloys in which magnesium predominates, comprising forming on the surface of the metal an adherent coating by subjecting the metal to the action of an aqueous solution containing a water soluble dichromatev and nitric acid and thereafter submentioned by way of illustration and not limitaan aqueous solution containing sodium dichromate and nitric acid for a period of from minute'to 2 minutesand thereafter subjecting .the coated metal to an aqueous solution of sodium dlcliromate saturated with magnesium fluoride for a period of from 5 minutes to one hour. the temperature of the second solution being maintained during treatment between C. and the boiling point of the solution.

3. A method of producing corrosion resistant coatings on magnesium and magnesium alloys in which magnesium predominates comprising forming on the surface of the metal an adherentcoating by subjecting the metal to the action' of an aqueous solution containing sodium dichromate and-nitric acid for a period of from minute to 2 minutes and thereafter subject ing the coated metal to an aqueous solution containing approximately 10 percent sodium dichromate saturated with magnesium fluoride for a period of from 5 minutes to one hour, the temperature of the second solution being maintained during treatment between 80 C. and the boiling point of the solution.

" ROBERT W. BUZZARD. 

